(1) Filed of the Invention
The present invention relates to an apparatus which is intended to apply to a wavelength division multiplex transmission apparatus, generates a dispersion different from one another according to a channel signal wavelength, and compensates a wavelength dispersion slope accumulated on an optical fiber transmission network.
(2) Description of Related Art
A light transmitter transmits a light pulse to a light receiver over an optical fiber in a conventional optical fiber communication system which transmits information using light. However, the wavelength dispersion of the optical fiber, which is also referred to as a xe2x80x9cchromatic dispersion,xe2x80x9d degrades the quality of a signal in the system.
More specifically, as a result of the wavelength dispersion, the propagation velocity of signal light in the optical fiber depends on the wavelength of the signal light in the optical fiber. For example, when a light pulse with a longer wavelength (such as a light pulse presenting red) propagates faster than a light pulse with a shorter wavelength (such as a light pulse presenting blue), its dispersion is referred to as a normal dispersion. On the other hand, when a light pulse with a shorter wavelength (such as a blue pulse) propagates faster than a light pulse with a longer wavelength (such as a red pulse), its dispersion is referred to as an abnormal dispersion.
Thus, if a signal light pulse includes a red pulse and a blue pulse when the signal light pulse is transmitted from a transmitter, the red pulse and the blue pulse are split from each other as the signal light pulse propagates over an optical fiber, and the individual light pulses are received by a receiver at different moments.
As another example of the light pulse transmission, when a signal light pulse having wavelength components continuously changing from blue to red is transmitted, since the individual components propagate at different speeds in an optical fiber, the width in time of the signal light pulse increases in the optical fiber, and a distortion is generated. Since any pulse includes components within a finite wavelength range, the generation of this wavelength dispersion is extremely general in the optical fiber communication system.
Thus, it is necessary to compensate the wavelength dispersion so as to obtain high transmission capability especially in a high speed optical fiber communication system. Consequently, a xe2x80x9creverse dispersion componentxe2x80x9d which adds a wavelength dispersion opposite to the wavelength dispersion generated in the optical fiber is necessary in the optical fiber communication system.
Some conventional apparatuses may be used as this xe2x80x9creverse dispersion componentxe2x80x9d. For example, a dispersion compensation fiber (DCF) has a specific cross section refractive index profile, provides a wavelength dispersion opposite to one generated in the conventional transmission line, and is used as the xe2x80x9creverse dispersion componentxe2x80x9d.
However, it costs high to manufacture the dispersion compensation fiber, and simultaneously, a relatively longer fiber is necessary for sufficiently compensating the wavelength dispersion generated in the transmission line. For example, a dispersion compensation fiber with a length of about 20 km to 30 km is necessary for completely compensating a wavelength dispersion generated in a transmission line of 100 km. Thus, the optical loss increases, and simultaneously the size increases.
A chirped fiber grating is used as the xe2x80x9creverse dispersion componentxe2x80x9d for compensating the wavelength dispersion as well. The fiber grating uses a phenomenon where the refractive index of germanium oxide used for doping the core changes as a result of ultraviolet irradiation, and forms a grating which changes the refractive index at a cycle of a half of a wavelength, and a longer wavelength component is reflected through a longer distance so as to propagate a distance longer than that of a shorter wavelength component as a result of gradually changing the interval of the grating in the lengthwise direction of the fiber. Thus, the chirped fiber grating also provides a light pulse with a reverse dispersion.
However, since the chirped fiber grating has a very narrow range in terms of the light to be reflected, it is impossible to provide a sufficient range for light including many wavelengths such as a wavelength division multiplex transmission signal. Though it is possible to cascade multiple chirped fiber gratings for a wavelength division multiplex transmission signal, the system becomes expensive.
In view of these conventional apparatuses, Published Japanese Translation of a PCT Application 2000-511655 (Japanese Unexamined Patent Application Publication HEI10-534450) and Published Japanese Translation of a PCT Application 2002-514323 (Japanese Unexamined Patent Application Publication HEI11-513133) propose an optical apparatus including a device called as a virtually imaged phased array (VIPA), for example.
This VIPA is a device which receives light having a respective wavelength within a continuous range of wavelengths, and generates output light continuously corresponding to the input light, and comprises parallel flat plates placed such that two reflection surfaces oppose to each other at a predetermined interval, the one reflection surface has light reflectance of 100%, and the other reflection surface has light reflectance smaller than 100% (about 98%) as disclosed in Japanese Patent Application Publication HEI 9-43057, for example.
A light incident window (a transparent area) for introducing light from the outside is provided on a part of the reflection surface with reflectance of 100%, and when light having a respective wavelength within a continuous range of wavelengths is obliquely introduced into the VIPA (between the parallel flat plates) from this light incident window, reflection is repeated between the parallel flat plates, and a partial light is released continuously from the multiple positions on the reflection surface with reflectance smaller than 100% to the outside of the VIPA.
Since the transmission light released at the multiple positions from the VIPA in this way travels while spreading radially at a certain angle, interference of light having a large number of different travel directions for the same wavelength occurs. Thus, only a component with a specific traveling direction depending on the wavelength is enhanced, light flux is formed, and consequently, it is possible to provide light having a respective wavelength within a certain continuous range of wavelengths toward directions different from one wavelength to another (namely, it is possible to provide the output light with an angle dispersion). In other words, the output light of the VIPA is spatially discriminated from one another having a different wavelength within the continuous range of wavelengths of the input light.
The technology described in Published Japanese Translation of a PCT Application 2000-511655 and Published Japanese Translation of a PCT Application 2002-514323 relates to technology using the characteristic of the VIPA so as to generate a wavelength dispersion. Specifically, the technology relates to an optical apparatus (a wavelength dispersion generating apparatus) constituted to return the light provided from the VIPA to the VIPA, and to generate the multiple reflection again in the VIPA.
Namely, this optical apparatus comprises a collimating lens 100, a cylindrical lens 200, a VIPA 300, a focusing lens 400, and a mirror 500 as shown in FIG. 18, input light from an optical fiber 600 is collimated by the collimating lens 100, the cylindrical lens focuses only one way of the light wave, the light enters into the VIPA 300 through a light incident window 301, the focusing lens 400 focuses (condenses) output light from the VIPA, the mirror 500 reflects the light, and the light is introduced into the VIPA 300 again.
The VIPA 300 and the focusing lens 400 are positioned such that the light proceeding from the VIPA 300 to the focusing lens 400 is parallel with and opposite to the light returning from the focusing lens 400 to the VIPA 300. The light proceeding from the VIPA 300 to the focusing lens 400 does not overlap the light returning from the focusing lens 400 to the VIPA 300.
Since the output light from the VIPA 300 is provided in different directions according to the wavelength in the optical apparatus constituted as described above, the focusing lens 400 focuses different wavelength components of the light at different positions on the mirror 500 (FIG. 18 shows a state where the light with a shorter wavelength is focused at a top part of the mirror 500, and the light with a longer wavelength is focused at a bottom part of the mirror 500). Thus, light with a different wavelength propagates to the VIPA 300 through an optical path with a different distance, and thus, a certain wavelength dispersion is generated.
Since the VIPA 300 actually provides multiple beams with the same wavelength and different interference orders (since the light beams with a different interference order are provided in a different direction), the focusing lens 400 focuses only light with a specific interference order on the mirror 500, and the mirror 500 is designed so as to reflect the only light with the specific interference order. The light returned to the VIPA 300 passes through the surface with the reflectance lower than 100%, is introduced into the VIPA 300, is reflected multiple times in the VIPA 300 again, and is provided from the light incident window 301 of the VIPA 300 through a path the same as the input path.
As described above, the VIPA 300 has a function of an angle dispersion as a diffraction grating does, generates a wavelength dispersion so as to compensate the wavelength dispersion, specifically has an especially large angle dispersion, and easily provides a practical reverse dispersion component.
However, there exist the following additional needs as the practical reverse dispersion component used for a wavelength division multiplex transmission system.
Namely, a wavelength dispersion of a general optical fiber for a practical application is not constant depending on the wavelength as shown in FIG. 19, for example, and often has a slight positive slope (the wavelength dispersion increases toward the positive direction as the wavelength becomes longer). This slope of the wavelength dispersion may be referred to as a wavelength dispersion slope or a second order wavelength dispersion.
For example, while a general single mode fiber (SMF) has a wavelength dispersion of +16.79 ps/nm/km for 1 km (see a broken line 700), a wavelength dispersion slope for 1 km is +0.057 ps/nm2/km, and a change of the wavelength dispersion is about +2 ps/nm2/km when a required range of wavelengths is 35 nm.
A solid line 800 and a broken line 900 indicate typical wavelength dispersion slopes of NZ-DSFs (non-zero dispersion shift optical fibers) in FIG. 19, and the solid line 800 and the broken line 900 respectively indicate Enhanced-LEAF (registered trademark) fiber (manufactured by Corning, abbreviated as E-LEAF hereafter), and TrueWave (registered trademark)-RS fiber (manufactured by Lucent, abbreviated as TW-RS hereafter).
The wavelength dispersion slope is not always positive (the wavelength dispersion increases toward the positive direction as the wavelength becomes longer), and changing the structural dispersion of a fiber may realize a negative dispersion slope.
Though the chart in FIG. 19 does not actually show straight lines, and the slopes of the wavelength dispersion (the wavelength dispersion slopes) are not constant in the strict sense, these third-order wavelength dispersions do not cause a problem at a transmission rate of about 40 Gb/s, and are negligible. In this way, it is desirable to provide a wavelength dispersion slope, namely a wavelength dispersion different depending on a channel signal wavelength, in addition to a wavelength dispersion as a practical reverse dispersion component.
A method of combining a VIPA and a branching filter such as a diffraction grating having a dispersion direction practically orthogonal to the dispersion direction of the VIPA as disclosed in xe2x80x9cCompensation of chromatic dispersion and dispersion slope using a virtually imaged phased arrayxe2x80x9d, (M. Shirasaki, OFC2001) or a specification disclosed in U.S. Pat. No. 6,343,866 is applicable as a method for generating a wavelength dispersion slope opposite to that of an optical fiber so as to compensate the wavelength dispersion. However, since a theoretical grating effect is hardly obtained in the diffraction grating due to a problem in a manufacturing process, there is such a problem that the loss of light is large.
The present invention is devised in view of these problems, and has a purpose of providing wavelength dispersion generation apparatus having a low optical loss and providing different wavelength dispersions depending on a channel signal wavelength, a multi-faced mirror used for the wavelength dispersion generation apparatus, and a manufacturing method for this mirror.
To attain the purpose above, the wavelength dispersion generation apparatus of the present invention comprises a virtual imaged phased array (abbreviated as VIPA hereafter) generator which receives input light having a respective wavelength within a continuous range of wavelengths and causes multiple reflection of the input light that splits the input light into a plurality of light beams which produce self-interference of the input light that forms an output light while the output light is spatially distinguishable from an output light formed on input light having any other wavelength within the continuous range of wavelengths, a lens which focuses the output light emitted from the VIPA, and a mirror which reflects the input light from the lens so as to return the light to the lens, where the mirror is constituted by a multi-faced mirror including multiple reflection surfaces in the traveling direction of the input light from the lens, and the reflection surfaces individually reflect the light with a different wavelength, and simultaneously individually have a different shape.
It is preferable that the reflection surfaces individually have a curved surface shape with a different curvature in the traveling direction of the input light from the lens, and the reflection surfaces are individually constituted by an optical film filter having a different transmission wavelength characteristic and a different reflection wavelength characteristic.
A multi-faced mirror used for the wavelength dispersion generation apparatus of the present invention is used for a wavelength dispersion generation apparatus for generating a wavelength dispersion, and reflecting input light having a respective wavelength within a continuous range of wavelengths, and comprises multiple reflection surfaces in the traveling direction of the input light, and the reflection surfaces individually reflect the light with a different wavelength, and simultaneously individually have a different shape.
With the present invention, since light with a different wavelength is reflected on the reflection surface with the different shape, an optical path difference depending on the wavelength is provided so as to generate the wavelength dispersion, and simultaneously, an apparatus with an optical loss lower than that of a conventional apparatus is provided.
The multi-faced mirror is manufactured following the steps below, for example.
(1) A first step of applying first ultraviolet curing resin with a light transmission characteristic on a substrate.
(2) A second step of bring a first die having a first sectional shape into press-contact with the first ultraviolet curing resin, and irradiating ultraviolet to harden the first ultraviolet curing resin.
(3) A third step of separating the first die from the first ultraviolet curing resin.
(4) A fourth step of forming a first optical film filter serving as a first reflection surface having a first transmission wavelength and a first reflection wavelength on the first ultraviolet curing resin.
(5) A fifth step of applying second ultraviolet curing resin with a light transmission characteristic on the first optical film filter.
(6) A sixth step of bring a second die having a second sectional shape different from the first sectional shape into press-contact with the second ultraviolet curing resin, and irradiating ultraviolet to harden the second ultraviolet curing resin.
(7) A seventh step of separating the second die from the second ultraviolet curing resin.
(8) An eighth step of forming a second optical film filter serving as a second reflection surface having a second transmission wavelength and a second reflection wavelength respectively different from the first transmission wavelength and the second reflection wavelength on the second ultraviolet curing resin.
In this way, since the multi-faced mirror is manufactured by repeating the individual steps, which comprise applying the ultraviolet curing resin, bringing the die in press-contact, irradiating ultraviolet, separating the die, and forming the optical film filter, depending on the number of reflection surfaces required for the multi-faced mirrors, the multi-faced mirror is easily realized.